For some time it has been recognized that phenol, especially when produced by the cumene process or by sulfonation of benzene, inherently contains some impurities which upon chlorination of the phenol give rise to objectionable color formation. See, for example. U.S. Pat. No. 2,864,869. Such impurities are present even in phenol which is U.S.P. grade, and are thought not to be separable from the phenol by ordinary fractional distillation. See, U.S. Pat. No. 3,029,292. Hence, a number of techniques have been developed for the purification of phenol which involve contacting the phenol with some chemical agent or heat treating it in the presence of a catalyst and thereafter recovering the phenol by fractional distillation. See, for example, U.S. Pat. Nos. 3,102,149 and 3,029,293 as well as those patents mentioned above. Upon subsequent chlorination, phenols treated in such a manner show a reduction in the typical dark reddish or brownish color which chlorination usually imparts to pentachlorophenol made from phenols produced by the cumene process or sulfonation of benzene.
However, such phenol purification techniques suffer from the disadvantages of elaborate and costly equipment and procedure. In other words, in order to obtain polychlorinated phenol such as pentachlorophenol free from colored impurities, heretofore it was necessary to perform a special purification on phenol already purified to U.S.P. grade, recover the phenol from such a special process, chlorinate the phenol and then recover the chlorinated phenol.
An improved process for the manufacture of polychlorinated phenols of improved (i.e., lighter) color is disclosed in a copending U.S. patent application assigned to the assignee of the present invention. As therein disclosed, chlorination of the phenol starting material is performed to obtain a mixture of chlorophenols containing from 1 to 3 chlorine atoms per molecule to phenol, at least 40% of the chlorinated phenol mixture being in the form of dichlorophenols. The reaction mixture is thereafter distilled to separate a light colored chlorophenol distillate from a residue containing dark color-forming impurities, and the distillate may be further chlorinated to form pentachlorophenol.
Certain impurities such as the chlorodibenzo-p-dioxins (commonly known as chlorodioxins) generally present in chlorinated phenols may be toxic to humans, possibly even in the low concentrations, e.g., about 2,000 p.p.m., present in technical grade pentachlorophenol. These chlorodioxin impurities are apparently formed during phenol chlorination and are thus not removed during special phenol purification procedures prior to chlorination. If the safety hazard of these compounds is established, then the removal of chlorodioxin to a safe level (i.e., below about 100 p.p.m.) will be necessary.